This depends on the magnifying device being used. In order to view the reticle and the object through a simple magnifier, the reticle is placed directly on the object plane. The base magnifiers are machined to accept a given diameter reticle. Simply select the reticle diameter that is accepted by the magnifier. The reticle in a direct measuring or pocket microscope is placed at an intermediate image plane and is scaled according to the power of the objective. These reticles are generally not interchangeable.
There are some eyepieces, however, which will accept reticles. In this case you would just match the reticle diameter to the reticle acceptance diameter of the eyepiece. Please note that reticles have different types of scales in order to accommodate different applications. Additionally, the correct eyepiece size must be selected to match the microscope. If a microscope has a removable eyepiece, then it can be exchanged with a similarly sized eyepiece that can accept a reticle. Note however that replacement eyepieces that can accept reticles are not necessarily the same length as the original eyepiece. This causes a height difference between two eyepieces for a binocular microscope, for example. If there is a large eye relief difference, you may have to also change the second eyepiece.
Another difference between magnifier and microscope reticles is the size requirements. The thickness for most magnifier reticles is larger than the thickness of a microscope reticle and will not be able to fit in a microscope eyepiece. Also, diameter tolerancing is an issue. The diameter tolerance of our microscope reticles is +0mm/-0.1mm, meaning that the diameter of the reticle will not be larger than the catalog specification. This assures that the microscope reticle will fit inside the eyepiece tubing. Our magnifier reticles have a diameter tolerance of ±0.05mm, meaning that the diameter of the reticle might be as much as 0.05mm larger than what we list in the catalog. As a result, there is a chance that a magnifier reticle will be too large to fit in the eyepiece tube, even though it matches the stated diameter reticle needed for that eyepiece.